1. Field
Embodiments of the present invention generally relate to flow sensors, and more particularly to systems and methods for compensating for transient thermal response of hot-wire anemometers resulting from a change in the mixture of gas being measured.
2. Description of the Related Art
Many medical ventilators use hot-wire anemometer flow sensors to measure flow rates both at the input (inspiratory) and output (expiratory) ports. Hot-wire anemometers are usually calibrated for specific gases and calibration parametric values are different depending on the gas type (e.g., air or oxygen). On the inspiratory side of a medical ventilator, anemometers with single gas calibration (e.g., air or oxygen) may be used in the appropriate gas conduits; however, at the expiratory side, the mixture of gas passing through an exhalation flow sensor varies based on breath phase and other factors.
Hot-wire anemometers operate with either constant current or constant temperature structure. They are customarily used as part of a Wheatstone bridge and electronic control circuits are used to maintain a fixed temperature across or current through the anemometer's filament. Changes in certain properties of the gas passing by the filament impact the heat exchange between the filament and the gas flow, thus leading to a change in the filament's temperature. Any deviation between the existing temperature and the reference temperature causes an imbalance in the bridge and the electronic control circuits respond by taking action to increase or decrease the current through the filament to adjust the temperature error. Notably, however, it takes some time (transient period) for the electronic temperature controller to respond and increase or decrease the filament current to adjust to the reference temperature. Thus, if a step change (mass flow rate) is generated in the composition of gas types in the mixture of gas passing through the exhalation flow sensor, a transient response (under or over estimation of the actual volumetric flow rate passing through the sensor) is observed.
In medical ventilators, accurate flow measurement is vital to the performance fidelity of pneumatic control systems, triggering and cycling event detection and reliable spirometry data. Thus, a real-time compensatory mechanism is needed to correct flow sensor measurements provided by hot-wire anemometer flow sensors during this transient period.